1. Field of the Invention
This invention relates to security systems and particularly to an anti-fraud alarm transmitting system from a protected area or monitored station to a central monitor via a communication line. This line may either be a special-purpose two-wire line, i.e. a line permanently connected to transmitting and receiving devices at both ends, or a subscriber's line to a telephone network.
2. Description of the Prior Art
It has been proposed to use telephone connections for transmitting an alarm signal, the signal being given by cutting off at least one "non-alarm" component at a preset frequency permanently transmitted by the alarm transmitting device from one station to a monitoring center comprising means for receiving the "non-alarm" component, and adapted to give an indication of alarm conditions, when the "non-alarm" component is not received. For example, such a system may be used on a burglar alarm system, and when an intruder is detected a transmitting station will cease transmitting the "non-alarm" signal. Staff at the monitoring center will note the resultant indication of alarm conditions and may notify security personnel or the Police, as is appropriate.
A known device of the aforementioned kind uses one or more frequencies which are in the telephone band from 300 to 3400 Hz, or outside the band in the case of a telephone connection including a subscriber's line to a telephone network. However, the known device can be effectively put out of action in fraudulent manner, e.g. by using an ordinary tape-recorder to record the transmitted "non-alarm" frequencies and retransmit them from the tape recorder to the monitoring center after disconnecting the original alarm transmitting device.
Another prior art alarm line security system includes a plurality of alarm sources which include intrusion detectors located in areas to be protected, each of which detectors provides an alarm indication responsive to the detection of an unauthorized entry of one of the protected areas. Each alarm source is connected to a separate alarm source monitoring circuit which includes a code generator which generates a selected sequence of code bits for transmission to alarm line monitoring circuits at a central monitoring location. The coded sequence of bits from each alarm source monitoring circuit are multiplexed for transmission to the central monitoring location over a single pair of transmission alarm lines.
Furthermore, each alarm source includes an associated alarm source monitoring circuit having a code generator and means responsive to the detection of an unauthorized entry, to modify code bits output by the code generator to indicate an alarm. The alarm line monitoring circuits at the central monitoring area also include a reference code generator which generates a sequence of code bits which is identical to that provided by the code generators associated with each alarm source.
The alarm line monitoring circuit also includes a code comparator circuit which accepts the coded bits transmitted to the central monitoring area over the alarm line from each alarm source and compares such bits with the reference code bits generated by the reference code generator at the central monitoring location. Under normal conditions (i.e., when all protected areas are secure), the sequence of bits received from the remote alarm sources will be identical with the sequence of bits provided by the reference code generator. However, whenever an alarm indication is provided by one or more of the alarm sources, the code transmitted from such sources will differ from the reference code. With the detection of a difference in one or more bits by the code comparator circuit, an alarm output signal is provided to enable an alarm to be registered at the central monitoring area.
In this last mentioned alarm transmission line security system, pseudo-random code generators are to be provided in all the protected areas and in the central monitor and the alarm signals are multi-bit code words which can take two forms according to whether the protected area is in the normal condition or in the alarm condition. Accordingly, the monitored stations comprise circuitry which is as complicated as in the central monitor and the difficulty of maintenance in these monitored stations along with the circuitry initial cost dictate against the use of such systems.